Alkylaluminoxanes, in particular methylaluminoxane, are becoming increasingly important as an essential constituent of a new generation of catalyst systems for preparing polyolefins (single site catalysts). These new catalysts consist essentially, as is already known from the classical Ziegler-Natta catalysis, of a transition metal compound as catalyst and the alkylaluminoxane as organoaluminum cocatalyst component. Transition metal compounds which are preferably used are cyclopentadienyl, indenyl or fluorenyl derivatives of metals in group IVa of the Periodic Table (IUPAC notation). In contrast to conventional Ziegler-Natta catalysts, such systems not only possess, besides high activity and productivity, the ability to control the product properties as a function of the components used and the reaction conditions, but they additionally make accessible hitherto unknown polymer structures having promising properties with regard to industrial applications.
Many publications have appeared in the literature, which deal with the preparation of specific polyolefins using such catalyst systems. However, a disadvantage in virtually all cases is the fact that to achieve acceptable productivities, a high excess of alkylaluminoxanes is required, based on the transition metal component (the ratio of aluminum in the form of the alkylaluminoxane to transition metal is usually about 1,000:1). Owing to the high price of the alkylaluminoxanes on the one hand, and, on the other hand, additional polymer workup steps (such as deashing steps) required in some cases, polymer production on an industrial scale on the basis of such catalyst systems would often be uneconomical. In addition, the solvent toluene often used for the formulation of alkylaluminoxanes, in particular methylaluminoxane, is increasingly undesirable both for reasons of storage stability of the formulations (strong tendency towards gel formation) and with regard to the application of the polyolefins finally produced.
A significant reduction in the amount of alkylaluminoxane required in relation to the transition metal component can be achieved by applying alkylaluminoxane to inert support materials, preferably SiO.sub.2 (J. C. W. Chien, D. He, J. Polym. Science Part A, Polym. Chem., Vol. 29, 1603-1607 (1991). Such supported materials additionally possess the advantage of being easily separated off in polymerizations in the condensed phase (preparation of high-purity polymers) or being able to be used as freeflowing powders in modern gas-phase processes, with the particle morphology of the polymer being able to be predetermined directly by the particle shape of the support. Furthermore, alkylaluminoxanes fixed on supports are, as dry powders, physically more stable than solutions having a comparable Al content. This is particularly true of methylaluminoxane which, as already mentioned, tends towards gel formation in toluene solution after a certain storage time.
A number of possibilities for fixing alkylaluminoxanes on supports have already been described in the literature.
EP 0 369 675 (Exxon Chemical) describes a process in which the immobilization of alkylaluminoxanes is achieved by reaction of an about 10% strength solution of trialkylaluminum in heptane with hydrated silica (8.7% by weight H.sub.2 O).
EP 0 442 725 (Mitsui Petrochemical) effects the immobilization by reaction of a toluene/water emulsion with an about 7% strength solution of trialkylalumminum in toluene in the presence of silica at temperatures of from -50.degree. C. to +80.degree. C.
A further alternative is offered by U.S. Pat. No. 5,026,797 (Mitsubishi Petrochemical) by reaction of prepared alkylaluminoxane solutions with silica (predried at 600.degree. C.) at 60.degree. C. and subsequent washing out of the proportion of alkylaluminoxane not immobilized by means of toluene.
Finally, U.S. Pat. No. 4,921,825 (Mitsui Petrochemical) describes a process for immobilizing alkylaluminoxane by precipitation from toluene solution by means of n-decane in the presence of silica.
These processes are sometimes technically complicated, since they comprise, inter alia, low reaction temperatures at the beginning, or multistage workup processes, thus resulting in yield losses with regard to the amount of aluminum used in the form of aluminum trialkyls. In addition, the space-time yield is sometimes considerably impaired by the obligatory use of relatively high amounts of solvent.
It is therefore an object of the present invention to overcome these disadvantages of the prior art and to provide an economical process by means of which alkylaluminoxanes can, without use of organic solvents, be fixed on inert support materials in high yield and homogeneity in a reproducible manner, with the particle morphology of the support being retained and the products being finally obtained as free-flowing powders.